EVOLUTION OF MODERN SURFACE FEATURES 47 



eruptive area, which subsided gradually beneath them, and they 

 spread thence in all directions. Along their edges to the north, east, 

 and south, they overlapped an eroded surface of the Nevadan 

 orogenic belt, damming valleys and ponding their waters. 



At about the same time, a different form of volcanism was be- 

 ginning along the Cascade Range. Eruptions of explosive violence 

 built up piles of andesitic lava, mudflows, breccias, and tuffs, which 

 were probably surmounted by volcanoes. These accumulations 

 formed a chain approximately along the axis of the present range, 

 from northern Washington southward through Oregon into Cali- 

 fornia, where it joined the chain of andesitic eruptions in the Sierra 

 Nevada. The andesitic volcanics had a much more complex origin 

 than the basaltic, and probably formed on a line of weakness that 

 was developing along the Cascade-Sierra Nevada trend (Waters, 

 1955a, pp. 709-710). 



In both the Cascade Range and Sierra Nevada, this line of weak- 

 ness was first manifested at the surface by eruptions only, but later 

 the rocks along it began to be raised by crustal forces. Unlike the 

 segment in the Sierra Nevada, however, that in the Cascades was 

 neither tilted as a block nor greatly faulted ; the faults that occur are 

 minor and local. Moreover, volcanic upbuilding went on hand in 

 hand with crustal uplifts so that basement rocks were not laid bare 

 by erosion, except at the north end. The cross section of the range 

 exposed in the gorge of the Columbia River exhibits both volcanic 

 upbuilding and complex arching (Hodge, 1938, pp. 839-886). 



Quaternary Events in Cascade Range. The modern Cascade Range, 

 from northern Washington to northern California, is crowned by a 

 score or more of great volcanic cones (Fig. 9), whose construction 

 apparently began as early as the Pliocene, but whose growth con- 

 tinued until recent times — if, in fact, it has yet ceased. The cones 

 were built upon a deeply eroded surface of the older andesitic vol- 

 canic rocks that form the greater bulk of the range, apparently after 

 a time of quiescence of some duration in late Tertiary or early 

 Quaternary time. North of the Columbia River the cones stand 

 singly, but farther south they have built up a massive range of 

 volcanic rocks on the eastern side of the earlier range. 



History of Columbia River. The Columbia River, whose sources 

 are in the Rocky Mountains on the east, enters the Pacific in the 

 midst of the volcanic province. Much of its drainage basin is blessed 



